There is a power conversion device that converts direct current power input from a direct current power supply such as a solar cell panel or the like into alternating current power and supplies active power to an electric power system of alternating current. For example, such a power conversion device is called a power conditioner. In the power conversion device, a reactive power compensation of the electric power system is performed when the input voltage from the direct current power supply is zero or low. For example, in the case where the direct current power supply is a solar cell panel, the operation of the reactive power compensation is performed in a time period such as night, etc., in which the power generation amount is zero.
If the electrical power of the power conversion device flows back into the direct current power supply side when performing the reactive power compensation, the electrical power is undesirably wasted; and there is a possibility of the undesirable occurrence of negative effects on the direct current power supply. Therefore, it is necessary to suppress the reverse flow of the electrical power into the direct current power supply when performing the reactive power compensation.
For example, there is a configuration that suppresses the reverse flow by cutting off the direct current power supply from the power conversion device by using a low-pressure air circuit breaker (Air Circuit Breaker: ACB), an electromagnetic contactor with an arc chute, etc. However, in the case where an ACB, an electromagnetic contactor with an arc chute, or the like is used, the exterior form of the power conversion device becomes large; and the power conversion device becomes expensive. On the other hand, in the case where a simple electromagnetic contactor is used, part damage may be caused when opening in the state in which the direct current flows; or a decrease of the reliability may be caused by an inrush current flowing in the direct current capacitor when engaging. Also, there is a method of inserting a diode in the direct current input as a method for suppressing the reverse flow; but this increases the wasteful power consumption accompanying the forward voltage drop and reduces the device efficiency. Also, the addition of a cooling mechanism for the loss of the diode, etc., are problematic.
Therefore, in a power conversion device that performs a reactive power compensation and the supply of active power, it is desirable to be able to suppress the reverse flow of the electrical power into the direct current power supply by using a simple and highly-reliable configuration.